A drop on demand inkjet printing system typically includes one or more printheads and their corresponding ink supplies. A printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a resistive heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the nozzle for the case of a thermal inkjet printhead. The droplets are typically directed toward paper or other print medium (sometimes generically referred to as recording medium or paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead. The electronic firing pulses allow the passage of current through the resistive heater. The pulses are typically very short, on the order of a microsecond, and are of sufficient voltage to raise the temperature of the resistive heater to several hundred degrees Centigrade very quickly in order to form a vapor bubble for drop ejection. However, if the firing pulses are unintentionally too long, they allow current at high voltage to pass through the resistive heater for a length of time that causes overheating, thereby damaging the heater.
In addition to thermal inkjet printheads that have an array of resistive heaters for vaporizing ink to form bubbles to power drop ejection, there are other types of inkjet printheads that include arrays of resistive heaters. For example, a thermal actuator printhead causes drop ejection by rapidly heating a flipper formed by two materials having different coefficients of thermal expansion so that the heat causes a rapid bending motion to eject a drop. Furthermore, some types of continuous inkjet printheads include an array of resistive heaters that cause a stream of ink from the nozzles to break off into droplets of controlled sizes for subsequent printing of an image or for deflection from the path to the ink receiver.
All such printheads, as well as other types having resistive heater arrays, are susceptible to damaging of a heater if its electrical pulse is inadvertently left on for too long. Typically the pulsewidth of the electrical pulse is set by a controller in the printer. Normally the controller very reliably sets the appropriate pulsewidth. However, a hardware or firmware glitch, for example, can cause the pulsewidth not to turn off at the proper time, thereby damaging one or more heaters as described above.
Consequently, a need exists for a protective circuit for the inkjet printhead that protects the resistive heaters against inadvertently long electrical pulses.